Production of colored images



March 18, 1952 L. R. KOLLER ET AL PRODUCTION OF COLORED IMAGES Filed 001:. 24, 1950- Fig.3.

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" Inventors:

Lewis RKoHer", Fer'ci El.WiHiams,

Then" Attorney.

March 18, 1952 Filed Oct. 24, 1950 ANTENNA 22 L. R. KOLLER ET AL 2,590,018

PRODUCTION OF COLORED IMAGES 2 'SHEETSSHEET 2 RE CE I VER l 14 COLOR VIDEO SYNCHRONIZER AMPLIFIER l 25 HIGH VOLTAGE SYNCHRONIZ/NG SWITCH/N6 SIGNAL UNI T SE PARA TOR HORIZONTAL VERTICAL SWEEP SWEEP GENERATOR GENERATOR E/ H2 L u Inve no ors Their Attorney Patented Mar. 18, 1952 PRODUCTION OF COLORED IMAGES Lewis B. Keller and Ferd E. Williams, Schenectady, N. Y.,. 'assignors to General Electric Company, a corporation of New York Application October 24, 1950, Serial No. 191,862v

13. Claims.

This invention relates t0. the production of colored images as in color television, color radar and the like. More particularly, it relates to a cathode ray tube for producing color: efiects and to the. colored light. producing screen employed therein.

It has been proposed heretofore to provide in a colored image producing cathode; ray tube a screen comprising a plurality of superimposed layers of different colored light producing phosphors and to regulate the potential of the exciting electron beam so that only-the desired phosphor layer is excited to. produce a particular color. This system of selective excitation of phosphor layers, as in any arrangement of: superimposed layers of electron energy absorbing materials, is based upon the fact that the, potential of an electron beam maybe adjusted to allow the electrons to penetrate to a certain depth in the media before giving up a substantial, part of their energy, which in the case of phosphors is manifested in a light efiect.

In order to provide a continuous phosphor layer by the usual methods of deposition, such as by allowing the particles to settlefrom a liquid suspension, layers are produced which are a number of particles thick. Ithas been found that in the system of such layers so formed, in order to present a continuous phosphor surface to the electron barrage, the electron voltages or potentials required to penetrate to the farthermost layer to produce the desired range of colors are excessively high and above those normally desirable in a commercial or home television receiver or in radar apparatus. The production of such high potentials also requires more equipment and a larger power supply. There is, further, a large amount of absorption and scattering of light between the particles in such gross screens which detracts from the brightness and resolution of colors produced.

Attempts have been made to produce thinner screens in order to lower the requisite electron potentials. As a practical matter, if screens are of such reduced thickness as to require electron potentials of reasonably low intensity, there are also voids between the particles of phosphor material through which electrons may pass without producing any luminescence or light in the particular layer. There is, consequently, no absorption of electron energy and light production at those particular points in the layer, the electron beam proceeding unhindered tothe next layer where an undesired color or one not corresponding to the beam signal voltage, is pro- 2. duced. The net result is that a fluorescent screen made up of layers having voids, does. n

give a faithful reproduction of the subject scanned in that it produces colors other thanv those natural to thesubject.

An object of this invention is.t.0 produce an improved colored image producing screen.

Another object is to provide a colored image producing screen in which the colored image is characterized by increased brightness.

A further object of the invention. is, to provide such a screen which will operate at relatively lower electron beam exciting potentials than heretofore.

A still further object is to provide such. a. screen which has enhanced voltage-brightness charac teristics.

Another object of the present invention is to provide. such a screen which provides increased sharpness of color separation.

Another object is. to produce such a screen in which the image is reproduced in purer colors.

Another object of the invention is to provide such a screen which will faithfully reproduce the true colors of an object.

A further object is to provide a cathode, ray tube for accomplishing the above objects.

A still further object of the invention is to provide a colored. image system. for the realization of the above objects.

Other objects will become. apparent and the invention better understood from a consideration of the following description and. the drawing in which Fig. 1 shows a cross-section of a portion of the colored image, producing: screen of the present invention; Fig. 2a is a plot of brightness versus electron. beam voltage: for a gross liquid-settled three color screen; Fig. 2b is a similar plot for a flotation type screen of the present invention; Fig. 2c is a similar plot for a flotation type three color screen of the present invention having between the layers thereof electron absorbin barriers; Fig. 31 is a spectograin plot of sucha screen at three: difierent electron beam potentials illustrating the purity of colors obtainable using the. present invention; and Fig. l shows a typical circuit in connection with which the presentscr'een may be employed.

It has been found that an improved colored image producing cathode ray tube screen may be readily produced.

It has further been found. that anflimproved cathode ray. tube screen for the productionof colored images may be made by providing a plu- 3 rality of mono-particle thick layers of different colored light producing luminescent materials or phosphors, the thickness of each layer being such that the maximum excitation is obtained with the electron beam to which it is sensitive. Preferably, the layers are separated by light transparent, electrically non-conductive barriers which are capable of absorbing electron energy.

The principle upon which this invention is based is that the penetration of electrons emitted from any source, such as an electron gun in a cathode ray tube, depends upon their velocity which varies as the square of their potential. With a given material to be penetrated, the-higher the impinging velocity or potential, the greater is the electron penetration. It is also known that an electron beam gives up most of its energy at a particular depth in the material. If, then, three layers of phosphors, each of which produces a difierent colored light upon excitation, are superimposed one upon the other to form a composite screen, a potential or voltage may be applied to electrons impinging on the screen to cause them to penetrate to any desired depth and then give up most of their energy. Depending upon the initial velocity or potential, this may be made to occurin the first, or succeeding layer and the light produced is mainly the color of the light produced by the phosphor layer in which most of the energy of the electron beam is lost with some color from other layers in which energy is absorbed. Thus, if all the energy from an electron beam is absorbed in a first, red light producin layer, the color emitted is red. If the beam is adjusted to penetrate to and lose most of its energy in a second blue light producing phosphor layer, the color is mainly blue with some red, depending on how much of the energy is absorbed in the first, red layer. Similarly, if the electron beam is adjusted to lose most of its energy in a third, green light producing phosphor layer, the resultant color is mainly green. The amount of red and blue color depends again on the energy absorbed in the first and second layers.

It isv obvious that in order to obtain similar brightness response to the electron beam throughout the expanse of any one phosphor layer, the layer should be of uniform thickness. At the same time it should be continuous and thin enough to enable selective excitation by electron beams of low intensity.

. A method of making such phosphor coatings is described in copending. application Serial No. 172,449, filed July 7, 1950, by Lewis R. Koller and assigned to the same assignee as the present application, the teaching of the above-cited application being included by reference in this application. Inthe above application it is taught that continuous, mono-particle thick continuous layers of powders, including phosphors, may be the excessive thickness of the layers necessitates very high values of electron energy. In general, the overall absorption of electron energy by monoparticle thick layers is about one-third that of ordinary multi-particle thick screens. At the same time the mono-particle thick layer deposited, as taught herein, afiords a maximum coverage of the area upon which it is deposited. This is to be distinguished from thin layers deposited by ordinary methods which have an excessive number of voids or gaps.

Representative examples of transparent screen materials which may be used in connection with the present invention are as follows. Typical red colored light producing phosphors, which may be used, among others, are zinc-cadmium sulphide, magnesium silicate, magnesium germinate, and magnesium phosphate; typical blue phosphors are zinc sulphide and calcium magnesium silicate, while typical green phosphors are willemite or zinc orthosilicate and magnesium aluminate. Using mono-particle thick layers of red, blue, and green phosphors which are, respectively, 2.8 microns, 6.1 microns, and'19.3 microns thick the layers may be excited by electron beams of lOjkilovolts, 18 kilovolts, and 32 kilovolts in that order. When the red, blue, and green layers are 0.5;, 4.1 1, and 29 thick they are excited by electrons having potentials of 6 kilovolts, l2 kilovolts, and 25 kilovolts. As a practical matter, the thicknessof the phosphor next to the base of the screen is immaterial so long as it is thick enough to absorb all the energy in any electron beam impinging upon it. In the present instance this is the green phosphor.

Mono-particle thick layers of phosphors are further of great advantage in that their extreme thinness and'smoothness makes possible a more abrupt change in color with change in voltage.

While screens made solely of mono-particle thick layers of phosphors have superior colored image producin characteristics, the purity of the colored light produced by any one layer when excited by an electron beam may be enhanced by placing light transparent, electrically non-conductive, electron energy absorbing barriers or layers between the phosphor lay r These lay rs serve to absorb a certain amount of the energy of the electron beam and may be so adjusted in thickness as to insure that any particular phosphor layer beyond the barrier and farther from the electron beam source is subjected to electrons having a potential with a definite lower voltage or potential limit. In other words, the barriers provide a certain critical voltage below which there I is 'no excitation of the succeeding phosphor layer.

placed on a base structure by forming a monoparticlev thick. layer of the powder on a liquid, passing the surface to be coated upward through the layer and drying. It is further taught that successive mono-particle layers may be produced by simply drying each preceding layer before adding another by the same method.

.. The use of continuous, thin phosphor layers one particle thick permits the excitation of a particle phosphor layer at substantially lower electron beam potentials than in the case of gross layers since the screen may be made of small particles which absorb just enough electron energy to give a maximum light effect and no more. This is in contrast to ordinary continuous screens in which While any light transparent, electrically non-conductive electron energy absorbing material may be used for the barriers described herein, finely divided mica and silica are preferred. Alkali silicates may also be employed. These materials are normally applied in the same manner as the phosphors though they may be applied by other means such as, in the case of silica, by the condensation of the vaporized material on the surface to be coated. In a mica'layer about 4.0 microns thick the energy loss for l5ikilovolt electrons is about 7.5 kilovolts while for 40 kilovolt electrons the loss is about 2.4 kilovolts. Silica is typically used in thinner layers than mica, down to about 0.1 thick. For such a layer the energy absorbed is somewhat less than that lost in the above-mentioned mica layer.

' In delimiting a certain critical voltage below which there can be .no excitation of a succeeding phosphor layer, the purity of color from. each such succeeding layer is enhanced. For example, with successive layers of red, blue, and green light producing phosphors exposed to the electron beam in that order, a barrier between the red and blue layers results in greater purity of the red color at low voltages. As the voltage is raised above the critical value necessary to enable it to pass the first barrier, the brightness of the blue color increases rapidly. The eifect of the barrier between the blue and green layers is to permit the attainment of maximum brightness in the blue layer relative to the red layer before there is any excitation of the green layer.

A partial cross-sectional view of a composite screen made up of various mono-particle thick layers is shown in Fig. 1. The composite phosphor screen is laid on glass base I which may be fiat or have any desired radius of curvature. Preferably, the red, blue, and green light producing phosphors are laid in inverse order on the glass base, that is, the green layer 2 is laid adjacent the base, the blue layer 3 next, and the red layer 4 next. Light transparent, non-conductive or insulating barriers 5 and 6 are preferably placed respectively between the red and blue and blue and green layers. Phosphor layers 3 and 4 are one particle thick and a particle size is selected for each layer which will permit the absorption of the proper amount of the energy of an electron beam to giv the desired brightness. Phosphor layer 2 need not be a mono-particle thick layer, but may be liquid-settled to a thickness which will preferably absorb all electron energy entering it. Preferably, though not necessarily, an

aluminum film l is placed over the outermost layer 4 of phosphor. This aluminum film serves to drain ofi any electrostatic charge which may tend to accumulate on the phosphor screen. It also serves to reflect light which otherwise would be diverted to the interior of the cathode ray tube of which the screen is a part, and reduces halation efiects.

The advantages of the present mono-particle phosphor layers, and particularly the present mono-particle layers having electron energy absorbing layers therebetween, over ordinary gross phosphor layers will be made clear by reference to Figs. 2a-2c. In these figures the brightness of various phosphor screens are measured by a photocell using a suitable filter for each color and plotted against the impinging electron beam potential on an oscillograph trace. The screen of Fig. 2a is a three color red, blue, and green phosphor screen with the green phosphor laid next to the glass base by the usual, liquidsettling method to a density of 4.5 mg. per cm. the particles averaging under 6; in size, and the blue, and redphosphors laid in the same manner successivelyabove it in that order. At electron beam voltages up at 25 kilovolts, as shown in Fig. 20., only the red phosphor emitted any color. No color trace was visible for the blue or green phosphors.

The screen of Fig. 2b was laid down in the same order as in Fig. 2a. However, only the green phosphor was formed by liquid-settling means to a density of 4.5 mg. per cm. for particles averaging less than 6 in size. The blue and red phosphor layers were laid down by the flotation method mentioned elsewhere herein and consisted of substantially continuous, monoparticle thick layers of particles averaging less than 6 in size. The advantage of the monoparticle thick layers over the gross layers of Fig.

201. will at once be seen. Whereas in the. gross layers of Fig. 2a, at voltages less. than 25 kilo volts only the red phosphor emitted any color. in the mono-particle layer screenof Fig. 2b, at.

13 kilovolts the blue phosphor emitted a color which was about 56 per cent as bright as the,

red color. Likewise, at 13 kilovolts the brightness of the green phosphor emission was about. 29 per cent that of the red. This means thatfor the present mono-particle phosphor layers, the electron beam, at relatively low voltages, penee trates the various phosphor layers to produce varied color effects. This is to be distinguished from the. gross layers of Fig. 2a for which at voltages less than 25 kilovolts the electron beam had not penetrated even the first red layer.

Fig. 20 represents similar data for a screenlike that of Fig. 2b except that silica barriers about 1 thick were placed between the red and blueand the blue and green mono-particle thick Itwill be noted that. Fig- 2c that the effect of the barrier was to displace the oscillograph traces somewhat to the right. Thus,- at 13 kilovolts the brightness of the blue. color emission was only about 37 per cent of that of the red emission and the brightness of the green color was only 13 per cent of that. of thered. As the electron beam potential increased, the: preponderance of red decreased being displaced by a blue emission which was increased more rapidly in brightness over the green by meansof the barrier. As the potential was stillfurther-increased, the green emission likewise increased more rapidly in brightness than when no barrier was present.

It will 'be appreciated then that the present mono-particle thick multi-layer screen permits the production of a plurality of colors at electron, beam potentials under 25 kilovolts whereas the gross layer screens will produce at such voltages only the color ofthe first layer impinged upon. The presence of electron energy absorbing barriers between the phosphor layers further improves the purity of the colors in that as the electron beam potential increases the preponderance of red emission .decreases.

Another beneficial efiect of the present barriers is that, should voids be present to a very slight extent between phosphor particles, the barrier layers absorb enough electron. energy to insure that a false color is not produced. In the absence of the barrier, an electron beam meantto excite a particular layer but which strikes a void. space between the configuration of the particles in the mono-particle thick layer proceeds unhindered to the next layer to excite a successive layer not intended to be excited. The result is that regions of one color are interposed with foreign colors which detract from the quality of the image reproduced. When the present barriers are employed, their thickness may be so adjusted that energy is absorbed in them to the. extent that the succeeding layer of phosphor is not excited except by a voltage which is enough higher so that undesirable interference of. colors is obviated.

The extreme sharpness of the color change. obtainable using the present mono-particle layers separated by electron-energy absorbing layers is in contrast to that produced by gross superimposed layers of phosphor material. Fig. 3 shows typical results obtained with mono-particle laymally of a density of 4.5 mg. per cm.

7" ersaveraging less than 6 in size, except for the green phosphor layer, when exposed to electon 'beams of various potentials, the phosphors being arranged in order with the green color producing material nearest the glass with overlying successive layers of blue and red phosphors. Under kilovolt excitation the energy-wavelength distribution is shown in curve 8, the light coming almost entirely from the red layer to give a pure red color. Curve 9 shows results under 21 kilovolt excitation with the light coming from both the red and blue layer, but predominately from the blue. At 45 kilovolts, as shown in curve 10, the light comes from all three layers, but mostly from the green. This spectral distribution is typical of that obtainable at even lower electron beam potentials as with the screen of Fig. 2c.

The grainsize of the phosphors making up the present layers determines the electron beam potentials at which they operate. For example, with red, blue, and green phosphor particle sizes of 2.8, 6.1, and 19.3 microns, respectively, the exciting electrons are 10, 18, and 32 kilovolts in that. order. With red, blue, and green monoparticle layers 0.5, 4.1, and 29 microns thick the corresponding electron voltages are 6, 12, and 25. In allcases the silica barriers used between phosphor layers were about two or three microns thick.

As:a matter of actual practice the thickness of the layer next to the glass surface is not of importance so long as it blocks the passage of all electrons. While this layer may be of monoparticle structure, it may also be laid down by any of the usual settling processes and is nor- Usually the phosphor layers are formed on the glass with the phosphor of the highest efficiency lowermost and the others superimposed thereon I in decreasing order of efficiency. In the present examples this order has been green, blue, and red with the electron beam striking the layers in inverse order.

In order that the means of applying the teachings of the present invention may be more readily understood, reference is made to Fig. 4 which shows schematically a color television receiving unit. It will be understood, of course, that this particular arrangement is shown not in a limiting sense, but merely for purposes of illustration. It will also be understood that the present screen may be used wherever it is desired to convert electrical pulses into color; typical examples being radar and television.

Reference is made to Fig. 4 for a typical example of the operation of the present colored image producing fluorescent screen when used in a cathode ray tube as part of a television system which for the most part is shown schematically.

Cathode ray tube H has an evacuated envelope l2 containing an electron gun l-3, a deflecting system M, semiconductive coating l5, and screen [6. Electrons emitted by cathode I! are controlled and focussed by control grid l8, attracted by first anode l9 and further accelerated toward screen 16 by semiconductive coating l5 which acts as a second anode. The electron beam is aimed at the desired point on screen i 5 by means of deflecting system M which comprises horizontal deflecting coil 20 and vertical deflecting coil 2|.

Considering the operation of the illustrative system as a whole, the incoming signal is intercepted by the antenna 22 and converted to an intermediate frequency by means of the high frequency oscillator in receiver 23. The signal is usually further amplified in an intermediate freequencyamplifier, not'shown, to a level suitablefor detection. 'After detection, the signal amplified in video amplifier 24 to a proper level and applied to the control grid I8 of cathode ray which supplies timing pulses to a high voltageswitching unit 29 which in turn furnishes color signals to the post deflection electrode 30 at screen it to produce the proper sequence of colors.

The use of a post deflection acceleration cath-v ode ray tube, as in the present example, is pre-' ferred because the focus and deflection of theelectron beam need not be changed to correspond.

to each value of the high voltage applied tothe screen. Of course, with the addition of proper equipment, the present system may be applied to any type of cathode ray tube as is well known to those skilled in the art. While the present invention has been describe with particular reference to a color television system, it will be appreciated that it may be applied to any equipment in which it is desired to give a precise varied color indication in 'a. cathode ray tube. Colored radar is an example of another application of the invention.

There is provided by the present invention a multi-layer colored image producing mmmes-j cent screen which is characterized by faithful. reproduction of colors at relatively low electron beam potentials and with a low voltage drop across the phosphor layers. The mono-particle thick multi-layer screen is further characterized by enhanced brightness and a minimum of in ray tube and a television system for utilizing:

mono-particle thick, continuous layers of ma:- terials producing different colored light, said; layers being separated by films which absorb a part only of any electron energy passing there-: through.

2. A cathode ray tube screen for. producing.

colored images comprising a plurality of uniform,

mono-particle thick layers of phosphors capablev of producing different colors under cathoderay excitation, said layers being separated by films which absorb a part only of any electron energy passing therethrough. 1

3. A cathode ray tube screen for producing colored images comprising a plurality ofuniform, mono-particle thick, continuous layers of differ ent colored light-producing material, said layers being separated by films which, absorb a part, only of any electron energy passing therethrough.

4. A cathode ray tube screen for producing colored images comprising a plurality of uniform, mono-particle thick, continuous layers of dif-' ferent colored light-producing material, said layers being separated by films consistin 01f silica for absorbing a portion only of any electron energy passing therethrough.

5. A cathode ray tube screen for producing colored images comprising a plurality of uniform, mono-particle thick, continuous layers of different colored light-producing material, said layers being separated by films consisting of mica for absorbing a part only of any electron energy passing therethrough.

6. A cathode ray tube screen for producing colored images comprising a plurality of uniform, mono-particle thick, continuous layers of different colored light-producing material, said layers being separated by films consisting of alkali silicate for absorbing a part only of any electron energy passing therethrough.

7. A cathode ray tube for producing colored images comprising an envelope containing electron beam producing means, means for controlling the potential of said beam, and an image screen comprising a plurality of uniform, monoparticle thick layers of phosphors capable of producing difierent colors under electron beam excitation, said layers being separated by films which absorb a part only of any electron energy passing therethrough.

8. A cathode ray tube for producing colored images comprising an envelope containing electron beam producing means, means for controlling the potential of said beam, and an image producing screen comprising a plurality of uniform, mono-particle thick layers of luminescent material, each layer being capable of producing a particular color under electron beam excitation, said layers being separated by films which absorb a part only of any electron energy passing therethrough.

9. A cathode ray tube for producing colored images comprising an envelope containing electron beam producing means, means for controlling the potential of said beam, and an image producing screen comprising a plurality of uniform, mono-particle thick layers of luminescent material, each layer being capable of producing a particular color under electron beam excitation, said layers being separated by films consisting of silica for absorbing a portion only of any electron energy passing therethrough.

10. A cathode ray tube for producing colored images comprising an envelope containing electron beam producing means, means for controlling the potential of said beam, and an image producing screen comprising a plurality of uniform, mono-particle thick layers of luminescent 10 material, each layer being capable of producing a particular color under electron beam excitation, said layers being separated by films consisting of mica for absorbing a part only of any electron energy passing therethrough.

11. A cathode ray: tube for producing colored images comprising an envelope containing electron beam producing means, means for control-' ling the potential of said beam, and an image producing screen comprising a plurality of uniform, mono-particle thick layers of luminescent material, each layer being capable of producing a particular color under electron beam excitation, said layers being separated by films consisting of alkali silicate for absorbing a part only of electron energy passing therethrough.

12. A color television system comprising a cathode ray tube comprising an envelope containing electron beam producing means, means for controlling the potential of said beam, and an image screen comprising a plurality of uni: form, mono-particle thick layers of luminescent materials capable of producing different colors under electron beam excitation, said layers being separated by films which absorb a part only of any electron energy passing therethrough.

13. A color television system comprising a cathode ray tube comprising an envelope containing electron beam producing means, means for controlling the potential of said beam, and an image screen comprising a plurality of uniform, mono-particle thick layers of luminescent materials capable of producing different colors under electron beam excitation, said layers being separated by films which absorb a part only of any electron energypassing therethrough.

LEWIS R. KOLLER. FERD E, WILLIAMS.

REFERENCES CITED The following references are of record in the file of this patent:

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